1996 Pontiac Bonneville Surging, dying engine

I have a 96 Bonnevile, 3.8 w/ auto. I washed the engine a few weeks ago and immediately the car started idling wildly, dying and not able to pull it's own weight. It was running extrememly rich while this was going on, even to the extent the catalytic was glowing from unburnt fuel. At least I think that's what it was, never had trouble with it before the wash.
Anyway, my mechanic has replaced the plugs, plug wires, coil packs, ignition module and throttle positiion sensor. Even tried another compatible computer. We got it to where it would run occassionally, but it inevitably goes back to surging, dying and won't run. I noticed there is a grey coupler that plugs into the end of the ignition module that has several wires. When I can get it to idle, I can tug or pull on those wires and the engine will surge up or down with the movement of the wires. My question: it looks like I may have to replace the engine wiring loom. How hard is this? I have never really done anything like that - is it something a mechanic should do? Second, how in the heck can spraying degreaser, detergent or rinsing those wires cause all this?